Not applicable.
The present invention relates to a process and adsorbent for the recovery of krypton and/or xenon from gas or liquid streams, and to an apparatus for use in an adsorption process.
The use of the noble gases krypton and xenon is expected to rise in the coming years. Krypton is primarily used in the global lighting industry, for example in long-life light bulbs and automotive lamps. Xenon has applications in the aerospace, electronics and medical fields. In the aerospace industry, xenon is used in ion propulsion technology for satellites. Xenon provides ten times the thrust of current chemical propellants, is chemically inert and can be stored cryogenically. This results in lower xe2x80x9cfuelxe2x80x9d weight so that satellites can accommodate more useful equipment. Xenon also finds applications in the medical market as an anaesthetic and in X-ray equipment, and in the electronics market for use in plasma display panels.
Krypton and xenon are produced by concentration from air. Since their concentrations in air are so small (krypton 1.14 ppmv and xenon 0.086 ppmv).large volumes of air must be processed to produce reasonable quantities of krypton and xenon. An issue of interest is the recycling of xenon from the air of operating rooms where it has been used as an anaesthetic.
In practice, krypton and xenon are reclaimed from the liquid oxygen portion of a cryogenic air distillation process. Since the volatilities of krypton and xenon are lower than that of oxygen, krypton and xenon concentrate in the liquid oxygen sump in a conventional air separation unit. This concentrated stream of krypton and xenon can be further concentrated by stripping some oxygen in a distillation column to produce xe2x80x9crawxe2x80x9d krypton and xenon. However, this xe2x80x9crawxe2x80x9d stream contains other air impurities less volatile than oxygen which have to be removed before pure krypton or xenon can be produced. In particular, the xe2x80x9crawxe2x80x9d stream contains carbon dioxide and nitrous oxide, both of which have low solubility in liquid oxygen and tend to freeze out during the concentration of krypton and xenon, resulting in operational problems. In addition, various hydrocarbons (C1 to C3) present in the liquid oxygen can concentrate during the stripping of oxygen to produce a liquid oxygen stream with dangerously high levels of hydrocarbons.
These problems may be addressed by the use of a xe2x80x9cguard adsorberxe2x80x9d, that is, an adsorber capable of adsorbing impurities from the liquid oxygen stream before the oxygen stripping step.
A number of U.S. patents (U.S. Pat. No. 4,568,528, U.S. Pat. No. 4,421,536, U.S. Pat. No. 4,401,448, U.S. Pat. No. 4,647,299, U.S. Pat. No. 5,313,802, U.S. Pat. No. 5,067,976, U.S. Pat. No. 3,191,393, U.S. Pat. No. 5,309,719, U.S. Pat. No. 4,384,876 and U.S. Pat. No. 3,751,934) describe krypton and xenon recovery processes where guard adsorbers are not used. These patents disclose various ways of reducing methane concentration in krypton and xenon by reducing reflux ratios in the raw distillation column.
U.S. Pat. No. 3,779,028 describes an improved method for recovery of krypton and xenon from a reboiler of an air separation unit. The oxygen-rich liquid which leaves the reboiler passes through an adsorber for the removal of acetylene and other hydrocarbons. There is no disclosure of the type of adsorbent used or of the removal of carbon dioxide or nitrous oxide. Oxygen and residual hydrocarbons are removed from the oxygen-rich liquid, for example using a hydrogen blowpipe, and the resulting secondary concentrate of krypton and xenon is vaporised and passed through an adsorbent, for example active charcoal, silica gel or molecular sieve. Separate krypton and xenon fractions may be collected from the adsorbent.
U.S. Pat. No. 3,768,270 describes a process for the production of krypton and xenon. A portion of the liquid oxygen from the reboiler passes through an adsorber for removal of acetylene and carbon dioxide. As in U.S. Pat. No. 3,779,028, the adsorbent used in the adsorber is unspecified and removal of nitrous oxide is not addressed. The oxygen and hydrocarbons that are not removed in the adsorber are subsequently removed by combustion with hydrogen. The resulting concentrate of krypton and xenon is treated as in U.S. Pat. No. 3,779,028.
U.S. Pat. No. 3,609,983 also describes a krypton and xenon recovery system. In this system, a liquid oxygen stream is passed through a pair of alternating guard adsorbers where acetylene and higher hydrocarbons are removed. The stream is then further purified by distillation. The hydrocarbons which are not removed in the guard adsorbers are catalytically combusted, and the resultant carbon dioxide and water are frozen out by heat exchangers. The stream is purified by a final distillation. This document discloses the use of silica gel as a guard bed adsorbent.
U.S. Pat. No. 3,596,471 also describes a krypton and xenon recovery process. The process employs a hydrocarbon adsorber for removal of hydrocarbons from a krypton- and xenon-containing liquid oxygen stream. The stream is then stripped of oxygen by contact with gaseous argon, residual hydrocarbons are burned and the combustion products removed, and the stream is distilled to afford a mixture of krypton and xenon. No disclosure is made of the type of adsorbent used or of carbon dioxide and/or nitrous oxide adsorption.
U.S. Pat. No. 5,122,173 also discloses a process for recovery of krypton and xenon from liquid oxygen streams. The process employs an adsorber for higher hydrocarbons and nitrous oxide, but the adsorbent material is not indicated.
U.S. Pat. No. 4,417,909 describes a process for recovering krypton and xenon from the off-gas stream produced during nuclear fuel reprocessing. Water and carbon dioxide are removed by adsorption at ambient temperature and at xe2x88x92100xc2x0 F. respectively, using molecular sieves. The water and carbon dioxide free stream is then passed through a bed of silica gel which removes essentially all of the xenon from the stream. The xenon is then recovered from regeneration effluent of the silica gel bed by freezing out in a liquid nitrogen cooled metal container. This art teaches selective xenon adsorption on silica gel.
U.S. Pat. No. 3,971,640 describes a low temperatures adsorptive process for the separation of krypton and xenon from a nitrogen-rich stream. The separation is carried out in an oxygen-lean stream to minimise the potential of explosions between oxygen and hydrocarbons. The krypton- and xenon-containing stream at 90 to 100 K is sent through a first adsorbent bed of silica gel to adsorb xenon, krypton and nitrogen. The effluent from the first bed is then sent to another bed which contains synthetic zeolite. Krypton, nitrogen, oxygen and hydrocarbons are adsorbed on the second adsorbent. Alternatively, the gases are adsorbed on one adsorbent only. The adsorbed gases are then desorbed by stepwise heating from 105 to 280 K, then to 650 K. This document thus teaches the use of silica gel as an adsorbent for xenon. No guard adsorbent is disclosed.
U.S. Pat. No. 4,874,592 also describes an adsorptive process for the production of xenon. As in U.S. Pat. No. 3,971,640, silica gel (or active carbon or zeolite) is used as a selective xenon adsorption agent. The concentrated xenon so obtained is purified by catalytic removal of hydrocarbons.
U.S. Pat. No. 5,833,737 describes an ambient temperature pressure swing adsorption process for the recovery of krypton from air. The key to the process is the use of hydrogen mordenite as the adsorbent selective for krypton.
U.S. Pat. No. 5,039,500 describes an adsorptive xenon recovery process which uses an adsorbent such as silica gel to selectively adsorb xenon and krypton from a liquid oxygen stream. The concentrated krypton and xenon stream is desorbed by heating and evacuation. The desorbed stream is then admitted to a low temperature solid-gas separating column to solidify and capture the xenon. No guard adsorbent is used in this process.
U.S. Pat. No. 4,369,048 and U.S. Pat. No. 4,447,353 teach methods for treating gaseous effluents from nuclear reactors. Radioactive krypton and xenon produced during nuclear fission must be captured and stored. In these documents, radioactive xenon is adsorbed at ambient temperature on a silver exchanged zeolite, while radioactive krypton is adsorbed on the same type of zeolite at lower temperature, about xe2x88x92140xc2x0 C. Water and carbon dioxide are pre-adsorbed on zeolite molecular sieves, and nitrogen oxides are also pre-adsorbed. All steps in their processes, with the exception of krypton adsorption, are carried out at near atmospheric temperature and pressure.
U.S. Pat. No. 5,039,500 discloses an adsorption process for the recovery of xenon from oxygen streams. The adsorbent used is silica gel. Xenon is collected by freezing out.
WO00/40332 discloses the use of Li and Ag exchanged X type zeolites for separating nitrogen from oxygen and comments that Li exchanged zeolites are somewhat selective for oxygen as compared to argon, whereas Ag exchanged zeolites showed no selectivity for oxygen over argon. It is said that Li Ag exchanged zeolites are adaptable for selectivity for oxygen over argon, although the degree of selectivity demonstrated is very small, and that Ag ions adsorb nitrogen strongly.
In a first aspect, the present invention provides a process for recovering xenon and/or krypton from an oxygen containing gas stream also containing xenon and/or krypton inert gas comprising selectively adsorbing the inert gas on a solid adsorbent and desorbing and collecting the adsorbed inert gas, wherein the adsorbent is a Li and Ag exchanged X type zeolite.
The term X type zeolite is used herein to embrace low silica X type zeolites. Typical X type zeolite may have an Si/Al ratio of 1.25 but low silica X type zeolites as known in the art may have a lower Si/Al ratio, e.g. 1.0-1.05. Thus the use of X type zeolites with Si:AI ratio""s of 1.0 to 1.5 at least is included within the invention.
Preferably, the adsorbent comprises silver and lithium exchanged X zeolite with a silver exchange level of 5 to 40% equivalents, for example about 20%. As is conventional, the silver exchange level as a percentage on an equivalent basis is given by:       Exchange    ⁢          xe2x80x83        ⁢    capacity    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    zeolite    ⁢          xe2x80x83        ⁢          Ag      +        ⁢          xe2x80x83        ⁢    in    ⁢          xe2x80x83        ⁢    equivalents        Total    ⁢          xe2x80x83        ⁢    exchange    ⁢          xe2x80x83        ⁢    capacity    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    zeolite    ⁢          xe2x80x83        ⁢    in    ⁢          xe2x80x83        ⁢    equivalents  
Preferably, the zeolite has a silicon to aluminium ratio of 1.0 to 2.5.
Preferably, the gas stream has a temperature of 90 to 303 K (more preferably 90 to 110 K) as it is passed in contact with the adsorbent. Preferably, the zeolite has a temperature of 120 to 398 K (more preferably 120 to 298 K) during desorption of the inert gas. Preferably, the gas stream has a pressure of 5 to 150 psig (34.4-1034 kPa) as it is passed in contact with the adsorbent.
Preferably, the second adsorbent has a particle size of 0.5 to 2.0 mm.
Preferably, oxygen is desorbed from the zeolite before the inert gas is desorbed from the zeolite. Preferably, oxygen is desorbed from the zeolite by a flow of oxygen-displacing gas. Preferably, the oxygen-displacing gas is an unreactive gas, more preferably the oxygen-displacing gas comprises one or more gases selected from the group consisting of nitrogen, argon and helium. In a preferred embodiment, the oxygen-displacing gas is cold nitrogen. The preferred temperature of the oxygen-displacing gas is 90 to 173 K. The preferred pressure of the oxygen-displacing gas is 5 to 150 psig (34.4 to 1034 kPa). The flow of oxygen-displacing gas is preferably co-current to the oxygen-containing gas stream flow.
The inert gas may then be desorbed from the zeolite by evacuation, purging, and/or thermal regeneration following the oxygen-displacing step.
The krypton and/or xenon may be desorbed from the zeolite by purging with a desorption gas. Preferred desorption gases comprise nitrogen, argon, helium, hydrogen or a mixture of two or more thereof. Preferably, the adsorbent has a temperature of 120 to 373 K during desorption of krypton and/or xenon. Preferably, the krypton and/or xenon is desorbed with a flow of desorption gas counter-current to the oxygen-containing gas stream (feed) flow. The preferred desorption pressure is 0.2 barg to 5 barg (5.1 to 128.3 kPa).
The krypton and/or xenon are recovered in a product gas stream and are suitably removed therefrom by freezing out. Alternatively, the product gas stream may be distilled to obtain pure krypton and/or xenon.
Thermal desorption may involve active heating, for example by heated rods or coils in the adsorption vessel or by external heating. Such active heating reduces the amount of required desorption flow to maximise the concentration of inert gas in the product stream.
Xenon is finding increasing use as an anaesthetic gas and as a neuroprotectant, but it is much more expensive than nitrous oxide and it is therefore highly desirable that it be recovered and recycled for future use. When the anaesthetic is exhaled by a patient it is naturally mixed with oxygen, nitrogen, carbon dioxide and water and also hydrocarbons derived from the equipment. In one preferred process according to the invention as described above, xenon is recovered from an oxygen and nitrogen mixture containing xenon at a concentration higher than that of xenon in atmospheric air, for instance in a process in which xenon is recovered from a mixture chiefly comprising oxygen and nitrogen, being or derived from exhaled gas or blood from a patient anaesthetised or neuroprotected using xenon. This is possible because although the silver and lithium exchanged zeolite has been used for separating nitrogen (more strongly adsorbed) from oxygen (less strongly adsorbed), xenon is more strongly adsorbed than nitrogen on this adsorbent. This is in contrast to argon, which is adsorbed similarly to oxygen, as discussed above.
We have found that the Henry""s Law constants (initial isotherm slope) at 303 K for relevant gases on these adsorbents is as shown in the following table:
Thus, when the adsorbent bed is exposed to a flow of a mixture of these four gases, nitrogen is initially adsorbed in preference to oxygen, but is eventually displaced from the bed by both krypton and xenon.
Alternatively, the invention may be employed in a process for recovering xenon and/or krypton from liquid oxygen containing xenon and/or krypton comprising vaporising the liquid oxygen to form a gas stream and recovering xenon and/or krypton therefrom by a process as described above.
More generally, the invention may be used in a process for recovering krypton and/or xenon from a liquefied gas stream containing one or more of krypton and xenon and one or more of carbon dioxide, nitrous oxide and hydrocarbon impurities, the process comprising:
passing the liquefied gas stream in contact with a first adsorbent capable of removing one or more impurities from the gas stream;
vaporising the liquefied gas stream to form a gas stream;
passing the gas stream in contact with a second adsorbent capable of removing krypton and/or xenon from the gas stream;
periodically desorbing krypton and/or xenon from the second adsorbent to form a product gas stream; and
periodically regenerating the first adsorbent, wherein the second adsorbent is a Li and Ag exchanged X type zeolite.
Preferably, the liquefied gas stream is an oxygen-rich liquefied gas stream, for example an oxygen-rich liquefied gas stream obtained by fractional distillation of air.
Preferably, the first adsorbent is silica gel. More preferably, the silica gel has a surface area of at least 650 m2/g. Preferably, the silica gel particle size is from 0.5 to 2 mm.
Preferably, the liquefied gas stream has a temperature from 90 to 110 K as it is passed in contact with the first adsorbent. Preferably, the liquefied gas stream has a pressure of 0 to 150 psig (0 to 1034 kPa) as it is passed in contact with the first adsorbent.
Preferably, the liquefied gas stream is vaporised at a temperature of 120 to 303 K.
The conditions for the recovery of the xenon and/or krypton from the gaseous stream are as described above in accordance with the first aspect of the invention.
In accordance with all aspects of the invention, typically, at least two beds of the inert gas adsorbent are used, such that the gas stream is passed in contact with a first bed of adsorbent whilst krypton and/or xenon and optionally oxygen are desorbed from a second bed of the adsorbent. The gas stream is then passed in contact with the second bed of the adsorbent whilst krypton and/or xenon and optionally oxygen are desorbed from the first bed of adsorbent. Waste gas from the gas stream after passing in contact with the first bed of adsorbent may be used for final purge and repressurisation of the second bed of adsorbent, and vice versa.
In embodiments where a guard bed is used typically at least two beds of the guard bed (first) adsorbent are used, such that the liquefied gas stream is passed in contact with a first bed of first adsorbent whilst a second bed of first adsorbent is regenerated. The liquefied gas stream is then passed in contact with the second bed of first adsorbent whilst the first bed of first adsorbent is regenerated. Regeneration may be carried out by evacuation or purging, but is preferably carried out thermally (thermal swing adsorption process), for example at a temperature of 298 to 423 K.
Regeneration of the guard bed is preferably carried out with a gas stream essentially free of the impurities adsorbed by the guard bed, e.g. a stream of nitrogen. Preferably, the gas stream flow is counter-current to the oxygen containing gas stream.
Alternatively, the system could comprise one set of adsorber beds. In this case, when the beds are being regenerated, the feed liquid is collected in a holding tank.
Once the beds have been regenerated, the collected liquid is then sent to the adsorbers. This system is desirable since it reduces cost by reducing the number of vessels and number of valves.
Additionally, the system could comprise only one adsorber bed. In this embodiment, a vaporized oxygen-rich stream is sent to an adsorber vessel that contains the guard adsorbent (silica gel or zeolites, like CaX) and the rare gas recovery adsorbent (AgLiX). In this case, feed liquid is collected in a holding tank during regeneration of the adsorbent. The desorbed product gas contains carbon dioxide and nitrous oxide which must be removed during further processing of the rare gas-enriched stream.
In a third aspect, the present invention relates to a process comprising adsorption of krypton and/or xenon on an adsorbent comprising silver and lithium exchanged X zeolite. Preferably, the process further comprises desorption of krypton and/or xenon from the adsorbent.
In a fourth aspect, the present invention relates to an apparatus for carrying out a process of adsorption and collection of a component of a feed gas, comprising:
a first adsorbent bed;
a second adsorbent bed downstream of the first adsorbent bed;
an upstream manifold positioned upstream of the first adsorbent bed;
an intermediate manifold positioned between the first and second adsorbent beds;
a downstream manifold positioned downstream of the second adsorbent bed;
a first inlet in the upstream manifold to control the flow of a feed gas across the first and second adsorbent beds;
a second inlet in the intermediate manifold to control the counter-current flow of a first regeneration gas across the first adsorbent bed only;
a third inlet in the intermediate manifold to control the co-current flow of a second regeneration gas across the second adsorbent bed only; and
a fourth inlet in the downstream manifold to control the counter-current flow of a desorption gas across the second adsorbent bed only.
Preferably, the downstream manifold contains a first exhaust outlet for feed gas and a second exhaust outlet for second regeneration gas. Preferably, the intermediate manifold contains a third exhaust outlet for desorption gas. Preferably, the upstream manifold contains a fourth exhaust outlet for first regeneration gas.
Optionally, the apparatus further comprises an accumulation vessel upstream of the upstream manifold for the accumulation of liquefied feed during passing of the first and second regeneration gases and the desorption gas across the adsorbent beds. This enables a single guard bed and krypton/xenon adsorbent bed to be used for krypton/xenon recovery without interrupting the flow of feed gas.
Optionally, an additional first adsorbent bed and an additional second adsorbent bed are connected to the upstream manifold, intermediate manifold and downstream manifold of the apparatus such that feed gas is passed across one first and second adsorbent bed while first and second regeneration gases and desorption gas are passed across the other first and second adsorbent bed, with periodic changes. This allows continuous krypton/xenon recovery with no need to accumulate feed gas whilst regeneration and desorption occur.